This invention relates to respirator therapy, and more particularly to a digital waveform generator for use in controlling the cycling of an automatic respiratory ventilator.
Respiratory therapy uses various devices for automatic ventilation of the lungs. These devices are typically used for either short-term assist, or prolonged artificial respiration. Since they substitute for the entire respiratory system, the devices designed for long-term mechanical respiration must serve several functions. Foremost of these is "ventilation," i.e., supply of oxygen and removal of carbon dioxide. This function is directly related to perfusion of the tissues of the body with oxygen and removal of carbon dioxide. Since long-term ventilation requires a tracheostomy (an artificial opening in the trachea), the functions of the nose (humidification and heating of incoming air) also must be provided.
Mechanical ventilation differs in one main respect from normal respiration. In artificial ventilation, the air is supplied by applying a positive pressure at the mouth, or tracheostomy, while in normal respiration, air is drawn in by a negative pressure inside the thorax. This difference leads to a number of undesirable effects of mechanical ventilation primarily involving the cardiovascular system. In particular, cardiac output may be reduced under certain flow conditions. It has been shown that the particular volume-time waveform, which is used during inspiration, may result in different effects on the cardiopulmonary system. A decelerating flow pattern, for example, has a lower peak pressure in the upper airways and in the alveoli, and a corresponding smaller effect on the respiratory system. An accelerating flow pattern, on the other hand, has a lower mean pressure and a smaller effect on cardiac output.
Several methods of classification of ventilators have been proposed. The most widely used method is based on the manner in which the changeover from inspiration to expiration is controlled. Three types are distinguished: pressurecycled, volume-cycled, and time-cycled. In each case, the changeover occurs when a preset pressure, volume, or inspiration time is reached. Less widely used classifications include: specification based on the "stability" or "flexibility" of minute volume, tidal volume and inspiratory flow in the face of changing patient resistance and compliance, and functional analysis of ventilators based on the power and force supplied. Some of the additional controls found on ventilators include respiratory rate, inspiratory flow rate, tidal volume, minute ventilation, and ratio of inspiration time to expiration time.
Electronic control has been used on ventilators in varying degrees in recent years. Of the 81 ventilators listed in Mushin et al. Automatic Ventilation of the Lungs, 2d ed., Oxford Blackwell, 1969, 15 ventilators use some means of electronic control. In most cases, electronic control is implemented to provide more accurate and repeatable control signals than gas-powered or electro-mechanical ventilators. One group of electronically controlled ventilators includes devices which determine respiration rate and ratio by varying both inspiration and expiration time. This approach is unsuitable from an engineering point of view since there is interaction between controls, and from an operator's point of view because the controls are ambiguous. For example, if one adjusts inspiratory time and expiratory time, it is possible to obtain a clinically invalid ratio of less than 1:1. Another group of ventilators have no ratio control. The ratio is derived from other controls, generally rate, tidal volume, and inspiratory flow rate. In some cases, either inspiratory or expiratory time is adjusted rather than rate. In either case, invalid ratios can be obtained and the actual ratio is unknown unless appropriate monitoring is available. A third group of ventilators provide a direct setting for respiratory rate and ratio. However, ventilators in this group, and the other groups discussed above, have not had the combined independence of controls, stability of operation for variations in patient resistance and compliance, and overall accuracy of all controls characteristic of the present invention.